Wood may be mechanically or chemically pulped. The chemical pulping processes normally employed are the sulphate, sulphite and soda pulping processes. The economics of these processes rely heavily on the recovery of chemicals employed in them.
In the sulphate process wood chips are digested in a solution of sodium hydroxide, sodium sulphide and possibly some sodium carbonate. In the sulphite process wood chips are cooked in the presence of sulphur dioxide and a bisulphite or a sulphite. In the soda process the wood chips are cooked in the presence of a solution of sodium hydroxide and possibly some sodium carbonate. The wood pulp is separated from the liquor. The liquor from the sulphate process and the soda process is normally referred to as black liquor. The present invention is concerned with the recovery of sodium hydroxide from black liquors derived from the soda process.
The specification of Australian Patent No. 486132 describes a method of recovering sodium hydroxide from black liquor which involves the following process steps:
1. concentrating the black liquor; PA1 2. mixing ferric oxide with the condensed black liquor; PA1 3. burning the mixture to produce sodium ferrite; PA1 4. submerging the sodium ferrite in hot water to form sodium hydroxide and a precipitate of ferric oxide; PA1 5. reusing the ferric oxide by mixing it with more condensed black liquor and repeating steps 3 and 4. PA1 (i) fluidising particles of an iron rich mixed oxide of alkali metal and iron in a fluidised bed furnace; PA1 (ii) burning in the fluidised bed furnace, an organic liquor derived from treatment of organic material with an alkali metal compound, to produce particles of alkali metal ferrite; PA1 (iii) recovering the particles of alkali metal ferrite from the fluidised bed furnace; and cooling them to produce cooled particles of alkali metal ferrite; PA1 (iv) mixing the cooled particles of alkali metal ferrite with an aqueous solution of alkali metal hydroxide at a temperature in excess of 80.degree. C. to form a more concentrated solution of alkali metal hydroxide and a precipitate of the iron rich mixed oxide of alkali metal and iron; PA1 (v) recovering the more concentrated solution of alkali metal hydroxide; and PA1 (vi) recovering the precipitate and feeding it to the fluidised bed furnace.
The specification of Australian Patent No. 519156 describes a process for recovering sodium hydroxide from black liquor which differs from the process described in Patent No. 486132 by the inclusion of a cold washing step. Sodium ferrite is washed in cold water to remove sodium chloride, sodium sulphate and other soluble impurities. The specification of Australian Patent No. 519156 also describes the use of a fluidised bed in the burning step.
The specification of Australian Patent No. 552973 describes a process similar to the process described in Australian Patent No. 519156 with the exception that it includes the step of agglomerating fines of ferric oxide and fines of sodium ferrite with black liquor before or during burning in a fluidised bed. The fines of ferric oxide are derived from the ferric oxide precipitated when sodium ferrite is added to warm water.
There are also a considerable number of Japanese patent applications that describe similar processes for recovering sodium hydroxide from black liquor. Some involve mixing the ferric oxide with the black liquor prior to combustion in a fluidised bed. Some involve combustion of the black liquor in a fluidised bed comprised of particles of ferric oxide or other transition metal oxide such as titanium dioxide. Some also involve combustion of black liquor in a fluidised bed comprised of another metallic oxide as well as ferric oxide. Sodium ferrite forms on the substrate oxide and the substrate oxide and ferric oxide are recovered when the agglomerate from the fluidised bed is added to hot water. The substrate oxides may be magnesia or alumina. However, only one patent specification describes the step of dissolving the sodium ferrite in an aqueous solution of sodium hydroxide. This is the specification of Australian Patent No. 599933. In a preferred form of this process the aqueous solution of sodium hydroxide has a concentration in the range from 10 to 150 gms per litre and more preferably 50 to 150 gms per litre. However, again the iron oxide recovered by the hydrolysis of the sodium ferrite is stated to be a mixture of hydrated iron oxide and unreacted iron oxide.